1. Field of the Invention
The present invention relates to a method and apparatus for reception of Bluetooth signals, and more especially, to a method and apparatus for reception of long-range signals in Bluetooth.
2. Background of the Related Art
The Bluetooth standard distinguishes devices by their so-called power class {[IEEE 802.15.1], [BT SIG 1.2], [BT SIG EDR]}. For each power class, a maximum output power (Pmax), a nominal output power and a minimum output power is specified as shown in Table 1.
TABLE 1PowerMaximum OutputNominalMinimum OutputClassPower (Pmax)Output PowerPowerPower Control1100 mW (20 dBm)N/A  1 mW (0 dB)Pmin < +4 dBmto PmaxOptional: Pmin2to Pmax2 2.5 mW (4 dBm)1 mW (0 dBm)0.25 mW (−6 dBm)Optional: Pmin2to Pmax3 1 mW (0 dBm)N/AN/AOptional: Pmin2to Pmax
The Bluetooth technology is intended to implement wireless personal area networks (WPAN). Therefore, the typical range of Bluetooth devices is expected to be limited to about 10 meters. Bluetooth devices according to power class 1, however, are capable to transmit over a range significantly larger than the so-called personal operating space (POS) of about 10 meters.
[IEEE 802.15.1]: the IEEE Std 802.15.1-IEEE Standard for Information technology Telecommunications and information exchange between systems Local and metropolitan area networks Specific requirements-Part 15.1: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Wireless Personal Area Networks (WPANs), 14 Jun. 2002.
[BT SIG 1.2]: Bluetooth SIG Specification of the Bluetooth System, Version 1.2, 5 Nov. 2003.
[BT SIG EDR]: Bluetooth SIG Specification of the Bluetooth System with EDR, Version 2.0, 4 Nov. 2004.
Sensitivity Performance in Bluetooth
In [BT SIG EDR], a reference sensitivity level of −70 dBm is given for an uncoded bit error rate (BER) of 0.0001 (0.01%). In FIG. 1 and FIG. 2, the uncoded BER versus SNR is shown for PI/4-DQPSK and D8PSK, respectively. For PI/4-DQPSK, about 14 dB SNR are needed to achieve an uncoded BER of 0.01%. For D8PSK, about 20 dB SNR are needed to achieve an uncoded BER of 0.01%. Additional SNR margin is needed to accommodate fixed-point implementation losses as well as losses introduced by radio front end impairments and non-ideal time and frequency synchronization. Therefore, about 25 dB SNR are assumed to achieve an uncoded BER of 0.01%.
Path Loss in Bluetooth
The signal power received by a Bluetooth device depending on the signal power transmitted by another Bluetooth device is given by Equation 1:PRX=PXT−Lpath−LFade+GTX+GRX  (1)with
PRX: received signal power
PTX: transmitted signal power
LPath: path loss
LFade: fade margin
GTX: received antenna gain
GRX: transmit antenna gain
The following assumptions are applied in Equation 2 and Equation 3:GTX=GRX=0 dBi  (2)LFade=8 dB  (3)
Therefore, based on Equation 1 and Equation 2, the path loss is given by Equation 4:LPath=PTX−PRX−8 dB  (4)The transmitted signal power under consideration (maximum signal power) is in Equation 5:PTXPTX,max=20 dBm (Power class 1 device)  (5)The received signal power under consideration (minimum signal power) is given by Equation 6:PRXPRX,min=NFloor+W+SNRRX+NFRX  (6)with
PTX,max: maximum transmit power
PRX,min: minimum received power
NFloor: noise floor due to thermal noise
W: noise bandwidth
SNRRX: signal-to-noise-ratio required for BER=0.0001 for D8PSK
NFRX: receiver noise figure
The noise floor due to thermal noise amounts to −174 dBm per Hz signal bandwidth. The signal bandwidth for Bluetooth technology equals 1 MHz. The receiver noise figure is assumed to be 20 dB.
The minimum signal power can now be computed by Equation 7:
                                                                        P                                  RX                  ,                  min                                            =                                                                    -                    174                                    ⁢                                                                          ⁢                  dB                  ⁢                                                                          ⁢                                      m                    /                    Hz                                                  +                                  1                  ⁢                                                                          ⁢                  MHz                                +                                  25                  ⁢                                                                          ⁢                  dB                                +                                  20                  ⁢                                                                          ⁢                  dB                                                                                                        =                                                                    -                    114                                    ⁢                                                                          ⁢                  dB                  ⁢                                                                          ⁢                  m                                +                                  45                  ⁢                                                                          ⁢                  dB                                                                                                        =                                                -                  69                                ⁢                                                                  ⁢                dB                ⁢                                                                  ⁢                m                                                                        (        7        )            
The maximum path loss based on maximum transmit signal power and minimum received signal power and fade margin based on Equation 4 is now given by Equation 8:
                                                                        L                                  Path                  ,                  max                                            =                                                P                                      TX                    ,                    max                                                  -                                  P                                      RX                    ,                    min                                                  -                                  8                  ⁢                                                                          ⁢                  dB                                                                                                        =                                                20                  ⁢                                                                          ⁢                  dB                                +                                  69                  ⁢                                                                          ⁢                  dB                                -                                  8                  ⁢                                                                          ⁢                  dB                                                                                                        =                              81                ⁢                                                                  ⁢                dB                                                                        (        8        )            
It follows that the maximum path loss for a Bluetooth device of power class 1 equals 81 dB. For power class 2 and power class 3, the maximum path loss amounts to 73 dB and 69 dB, respectively.
On Transmission Range in Bluetooth
The path loss depending on the transmission range for line-of-sight (LOS) conditions in a Bluetooth network is given by Equation 9:
                              L          Path                =                  20          ⁢                      log            ⁡                          (                                                                    2                    ⁢                    Π                                    λ                                ⁢                R                            )                                                          (        9        )            or by Equation 10 approximatelyLPath=40+20 log(R)  (10)with
R: transmission range in [meters]
λ: wavelength of transmission signal
The path loss depending on the transmission range for non-line-of-sight (NLOS) conditions in a Bluetooth network is given by Equation 11:
                              L          Path                =                              36            ⁢                          log              ⁡                              (                                                                            4                      ⁢                      Π                                        λ                                    ⁢                  R                                )                                              -                      46.7            ⁢                                                  ⁢            dB                                              (        11        )            or Equation 12 approximatelyLpath=25.3+36 log(R)  (12)
Equation 9, by Equation 10, Equation 11 and Equation 12 are visualized in FIG. 3. It follows that for a maximum pathloss of 81 dB, a maximum transmission range Rmax of 113 meters (in large office) is achieved (LOS conditions) using a power class 1 device while ensuring reliable communication. For power class 2 and power class 3, 18 meters (in small office) and 11 meters (POS) are achieved, respectively.
On Multipath Propagation in Bluetooth
In Bluetooth, the symbol rate equals 1 Msps while the symbol duration TSymbol equals 1 μs (1000 ns). According the radio propagation theory, a radio frequency signal propagates 300 m in 1 μs (3e8 meters per second). The maximum echo delay (1st versus 2nd echo) based on the maximum transmission range is given by Equation 13:
                                                                        D                max                            =                                                                    T                    Symbol                                                        300                    ⁢                                                                                  ⁢                    m                                                  ·                                  R                  max                                                                                                        =                                                                    1                    ⁢                                                                                  ⁢                    µs                                                        300                    ⁢                                                                                  ⁢                                          m                                                                      ⁢                113                ⁢                                                                  ⁢                                  m                                                                                                        =                              377                ⁢                                                                  ⁢                ns                                                                        (        13        )            It follows that for a maximum transmission range Rmax of 113 meters a maximum echo delay of 377 ns is obtained.
For power class 2 and power class 3, 60 ns and 37 ns are obtained, respectively.
Multipath propagation results in inter-symbol interference (ISI). The amount of ISI introduced depends on the number and power of all echo paths following the first arriving path.
Using the result from Equation 13, one gets a maximum ISI percentage shown in Equation 14:
                                                                        ISI                max                            =                                                D                  max                                                  T                  Symbol                                                                                                        =                                                377                  ⁢                                                                          ⁢                  ns                                                  1                  ⁢                                                                          ⁢                  µs                                                                                                        =                              37.7                ⁢                %                                                                        (        14        )            
For power class 2 and power class 3, 6% and 3.7% are obtained, respectively.
The ISI is modelled as an echo path having a relative power (with regards to the first arriving path) equal to ISImax. With that assumption, a worst-case multipath channel profile with a 1 st (obstructed) path @ 0 dB w/delay of 0 samples and a 2nd path (echo) @ 10*log10(0.377)=−4.24 dB w/delay of 1 sample (1 μs).
The 2-path multipath propagation model for Bluetooth long transmission range applications is shown in FIG. 4. A large office scenario for Bluetooth long transmission range applications is shown in FIG. 5.
Impact of Multipath Propagation on Bluetooth Demodulation Performance
In FIG. 6, FIG. 7, FIG. 8, and FIG. 9, the simulated impact of multipath propagation on Bluetooth EDR demodulation performance is shown.
For the 2-path multipath propagation model, the power of the second path is varied relative to the first arriving path. For the exponential multipath propagation model, the RMS delay spread is varied.
In FIG. 6, one can see that even for Pi/4-DQPSK and a very small second path such as −15 dB (10 log10(0.0313)), there is a degradation exceeding 3 dB already. For path larger than −9 dB (10 log10(0.125)), successful demodulation is no longer possible independent of the SNR.
In FIG. 7, one can see that even for D8PSK and a very small second path such as −15 dB (10 log10(0.0313)), there is a degradation exceeding 12 dB (!) already. For path larger than −15 dB, successful demodulation is no longer possible independent of the SNR.
In FIG. 8, one can see that for Pi/4-DQPSK and an RMS delay spread>250 ns, there is a degradation exceeding 3 dB.
In FIG. 9, one can see that for D8PSK and an RMS delay spread>200 ns, there is a degradation exceeding 3 dB.
It was also shown that even for very moderate multipath propagation, no reliable data transmission using Bluetooth technology is possible. That is due to the inter-symbol interference (ISI) introduced by multipath propagation. Current (state-of-the-art) Bluetooth receivers are not capable of mitigating the unfavorable impact of ISI on the data demodulation in Bluetooth.
It is concluded that with current (state-of-the-art) Bluetooth receivers, no reliable data transmission is possible with regards to transmission ranges provided the transmission power of power class 1 devices.